The present invention relates to carbon steel wire for high tensile strength applications. The usual composition for this comprises alloying elements (herein defined as those elements that are present in an amount of at least 0.05%) among which the carbon is present in an amount ranging from 0.4 to 1.4%, manganese from 0.1 to 1% and silicon from 0.05 to 1%, the remainder being iron and impurities (herein defined as those elements that are in an amount of less than 0.05%), all percentages of this disclosure being percentages by weight.
By "wire" is meant here any elongated form, irrespective of the cross-sectional shape, the latter being circular in general, but the latter can also have another form, such as rectangular, with a width-to-thickness ratio ranging e.g. from 1 to 20, or any form. In such cases, the diameter of the circle having the same cross-sectional area will be considered here as the "diameter" of the wire.
The high tensile strength will in general have been obtained by cold working a pearlitic steel microstructure, preferably by drawing, but this can also have been obtained e.g. by cold rolling or a combination thereof with a preceding cold drawing operation.
It is known that steel of the composition above must not be cold drawn or worked into wire to such high tensile strength that this would result in unsufficient ductility for supporting bending and torsional loads. In dependence on the diameter, there is a tensile strength limit above which special care must be taken. This limit is higher for thin final diameters than for thick ones. This limit in function of the diameter is given by the formula (R.sub.m being the tensile strength limit in N/mm.sup.2 and d being the wire diameter in mm): EQU R.sub.m =2250-1130 log d (1)
which, in a tensile strength-versus-diameter diagram, shows a line, the "line of special care", above which there is the field of high tensile strength.
It is to the wire in this field of high tensile strength that the invention applies. In this field the wires can rather easily pass the current tests on ductility for axial loads, but the problems become more difficult when bending and torsional ductility tests are involved. For the wires having a tensile strength R.sub.m above a given line, called here the "problem-line", given by the formula EQU R.sub.m =2325-1130 log d (2)
the percentage of rejections in these bending and torsional tests become excessive. The difficulty is, that among wires that usually successfully passed the ductility test under axial load, there is a part that passes the bending and torsional ductility tests and another part that does not, and that the reasons of this different behaviour are unknown.
This puts a severe limit to the tensile strength to which the wires can be processed, at least for steel wire called for use under non-axial loads, when the wire will have to be deformed into the final product, such as the assembling into a steel cord, or when the wire in the final product is loaded as such, as in springs, bead wire, hose reinforcement wire, steel tire cord, conveyor belt cord and the like.
In order to minimize the rejection figures, and to be able to exceed the above problem line, we have tried in the sense of adding alloying elements, but the random and unpredictable character of rejections in the bending and torsional ductility tests remained. As a consequence, our attempts to minimize the rejection figures have been limited to conducting the patenting heat treatment operation in a careful way for obtaining the finest and most adequate pearlitic microstructure, and by drawing the wire very carefully, by adapting the speed and reduction per drawing-die in order to minimize microstresses and microcracks which might be the reason of this random behaviour in the bending and torsional ductility tests.